This invention relates to a thickness shear quartz crystal oscillator which can be made small and light and which has excellent electrical characteristics.
Generally, an AT-cut thickness shear quartz crystal oscillator exhibits a frequency-temperature characteristic which is represented by a three-dimensional curve and is stable against temperature variation. Thus, it is used as a reference frequency signal source in various electronic devices such as communication systems and electronic watches.
However, when used practically, the AT-cut thickness shear quartz crystal oscillator encounters various problems. To reduce the size of the oscillator without degrading its frequency-temperature characteristic so that the oscillator may be placed in a small casing of, for example, an electronic watch, the following problem will arise.
A known thickness shear quartz crystal oscillator is made of a quartz crystal plate which has a rectangular cross section and which extends in the direction of its X axis. The oscillator vibrates in a direction which is parallel to the X axis. To make the oscillator small, the crystal plate is cut at an angle of 34.degree.45' to 35.degree.20" (hereinafter called "cut angle") and is provided with an inclination angle of 2.degree. to 16.degree.. The cut angle and the inclination angle will be defined as follows.
Suppose the X-Z plane defined by the X and Z axes of the crystal plate is rotated about the X axis and that Y' and Z' axes are imagined which incline at the same angle to the Y and Z axes, respectively. Then, the "cut angle" is the angle at which X-Z' plane defined by the X axis and the imaginary Z' axis inclines to the X-Z plane. The "inclination angle" is the angle at which the side faces of the crystal plate incline when one of the major surfaces of the crystal plate which are parallel to the X-Z' plane is displaced in the direction of the imaginary Z' axis.
If cut at said cut angle and provided with said inclination angle, the crystal plate has its width reduced or it becomes shorter in the direction of the Z' axis, without deteriorating its own frequency-temperature characteristic which is represented by a three-dimensional curve with an inflection point at about the room temperature. The above-mentioned method of shaping a crystal plate, however, fails to make the plate shorter in the direction of the X axis. That is, the method cannot reduce the length of the crystal plate, for the following reason.
As disclosed in "30th Annual Symposium on Frequency Control--1976" pages 196-201, an AT-cut quartz crystal oscillator having a cut angle of 35.degree.30', a length a along the X axis and a thickness b along the Y axis will have its frequency-temperature characteristic changed if the length a is reduced a little so that the ratio of a to b decreases from 2.935 to 2.745. More specifically, the inflection point will appear at a temperature of 60.degree. C. to 80.degree. C. That is, if the length of an AT-cut quartz oscillator is reduced, the inflection point will appear at a temperature far higher than the room temperature, and the three-dimensional curve showing the characteristic will become almost two-dimensional in the vicinity of the room temperature. If this happens, the quartz crystal oscillator cannot be used practically.
An object of this invention is to provide a thickness shear quartz crystal oscillator which can be made small and light and whose length can be reduced without deteriorating the electrical characteristics.